An Arkansas dad charged with killing his 14-year-old daughter’s accused sexual predator is now running for sheriff … saying his fight for justice showed him exactly how law enforcement drops the ball.

Aaron Spencer — charged with fatally shooting 67-year-old Michael Fosler last year — kicked off his run for Lonoke County Sheriff in a Facebook video … calling out “failures” in law enforcement and the courts.

Court docs say 37-year-old Spencer — a military veteran — was charged with second-degree murder last October for allegedly killing Fosler — who was already facing multiple sex crime charges against Spencer’s daughter — after finding him in a truck with her.

According to the Northwest Arkansas Democrat-Gazette, the girl was reported missing about 1 AM on October 8 last year … responding deputies learned en route Spencer had found his daughter and Fosler in a truck … before deputies arrived at the scene, a confrontation ensued and Fosler was killed.

KTHV-TV reports Fosler had been facing more than 40 child sex crimes charges involving Spencer’s 14-year-old daughter – including sexual assault and sexual indecency with a child. The Arkansas Times reports the girl was 13 at the time of the fatal incident.

After his arrest, Spencer’s wife Heather started an online fundraiser, saying their underage daughter had been “targeted, groomed, and ultimately raped by the boyfriend of a family friend.” Fosler was reportedly free on $50K bond.

Spencer himself was released on surety bond after the shooting … his next court date is in December for a pre-trial hearing … with his trial set to kick off in January.